In pipelines delivering produced crude oil, parafin coating of the pipeline is a potential problem. Parafin, wax or asphaltene may coat the interior of the pipeline, thereby placing a narrow constriction in the pipeline and reducing its flow capacity. The coating material will be described as a parafin coating but the term will be understood to include other constituents of the produced oil which coat the pipe to form a constriction in the pipeline.
Perhaps an example will describe the situation readily. Assume that several deep wells produce oil from significant depth at an elevated temperature. Assume that the wells are below an offshore platform which is connected with a storage facility that is located many miles away. An underwater pipeline extends from the producing wells to the storage facility. Assume further that the water temperature is substantially cooler than the produced oil. As a matter of fact, the water can be quite cold as would occur with offshore wells located in the North Sea, near Alaska, or in other northern latitudes. In that instance, a very substantial temperature drop is experienced by the oil and this causes the heavier molecules to thicken and form a parafin coating in the pipeline.
Molecules of many different weights may deposit into the coating material. Those which are extremely heavy molecules more readily stick while the lighter molecules may dissolve from the coating. There is a tendency for lighter molecules to dissolve into the liquid leaving only heavier molecules, and over a period of time, the coating can become hard. Typically, this occurs with the greatest hardness adjacent to the pipe, and the newer strata of the coating are usually much lighter and also softer. In other words, the coating will vary in hardness.
Pigs have been used to remove parafin. There are several problems that relate to parafin scraping from a pipeline. First of all, pigs fabricated with metal bodies or with steel mandrels supporting rubber scrappers run the risk of breaking up in the pipeline. While it may be rare, nevertheless, it does occur and in that instance, the broken pieces of the steel member are carried along the pipeline and often destroy downstream equipment including pumps. This creates severe damage and requires expensive repairs.
There is another limitation to parafin scraping of the coated material in a pipeline. The coating is made of molecules of different weights and therefore has variations of hardness. If the coating is unsuccessfully scrapped, it tends to pack the parafin and form a harder coating, typically by squeezing only the lighter portions. As an example, it may remove most of the lighter portions but it may leave a much harder film coating in the pipe. At that juncture, the coating hardness approaches that of candle wax and is extremely difficult to remove.
Removal at this stage is not readily accomplished. Solvents which successfully remove parafin cannot be easily used in pipelines which are extremely long because of unreasonable cost. Thus, the accumulation of parafin as hard a candle wax is highly undesirable. This hard coating can regretably be left in a pipeline by scrapping improperly without dislodging a significant portion of material in the pipeline. The present apparatus sets forth a scrapping pig for a pipe suspected of having parafin coating in the pipe. It has the advantage of providing a unitized pig body formed of elastomeric materials. The materials are formed into a central column having a number of discs thereabout. The front or leading disc is made stronger, harder and stiffer than the remaining discs. The front disc is undergage while the softer discs are overgage. The softer discs accomplish sealing and scraping and yet they are soft thereby flexing to assure that dislodged particles of wax can then float down the pipeline. The softer (sealing) discs are more easily deflected as will be described thereby enabling more certain contact with the pipeline for cleaning parafin. Thus, the unitized pig of the present disclosure accomplishes paraffin removal.
While the foregoing speaks generally of problems encountered in pipeline paraffin removal, the preferred embodiment sets forth a more specific disclosure of a unitized pig for accomplishing paraffin removal, and other objects and advantages will become more readily apparent on consideration of the below written specification accompanied with the drawings attached hereto.